432 



THE STEAM-ENGINE. 



driven to the bottom of the cylinder. During its descent, the valves G, I, and 

 E, remain open. At the moment it arrives at the bottom of the cylinder, all 

 these three valves are closed, and the valve H opened. The steam which 

 fills the cylinder above the piston is now permitted to circulate below it, by the 

 open valve H and the piston being consequently pressed equally upward and 

 downward, will be drawn upward as before by the preponderance of the pump- 

 rods at the opposite end of the beam. The weight of these rods must also be 

 sufficiently great to draw the air-pump piston N upward. As this piston rises 

 in the air-pump, it leaves a vacuum below it, into which the water and air col- 

 lected in the condenser will be drawn through the valve M, which opens out- 

 ward. When the air-pump piston has arrived at the top of the barrel, which 

 it will do at the same time that the steam-piston arrives at the top of the cyl- 

 inder, the water and the chief part of the air or other fluids which may have 

 been in the condenser, will be drawn into the barrel of the air-pump, and the 

 valve M being closed by its own weight, assisted by the pressure of these flu- 

 ids, they cannot return into the condenser. At the moment the steam-piston 

 arrives at the top of the cylinder, the valve H is closed, and the three valves 

 G, I, and E, are opened. The effect of this change is the same as was al- 

 ready described in the former case, and the piston will in the same manner and 

 from the same causes be driven downward. The air-pump piston will at the 

 same time descend by the force of its own weight, aided by the weight of the 

 plug-frame attached to its rod. As it descends, the air below it will be gradu- 

 ally compressed above the surface of the water in the bottom of the barrel, un- 

 til its pressure becomes sufficiently great to open the valves in the air-pump 

 piston. When this happens, the valves in the air-pump piston, as represented 

 on a large scale in fig. 9, will be opened, and the air will pass through them 



Fig. 9. 



above the piston. When the piston comes in contact with the water in the 

 bottom of the barrel, this water will likewise pass through the open valves. 

 When the piston has arrived at the bottom of the air-pump barrel, the valves in 

 it will be closed by the pressure of the fluids above them. The next ascent 

 of the steam piston will draw up the air-pump piston, and with it the fluids in 



